In the related art, various light emitting element driving apparatuses for driving light emitting elements such as light emitting diodes (LEDs) have been developed.
Recently, in-vehicle displays, which are display devices mounted on vehicles, are in widespread use. In such in-vehicle displays, it is necessary to change luminance between a vehicle traveling during the daytime and a vehicle traveling during the nighttime or in a tunnel. Specifically, it is necessary to increase the luminance during the daytime traveling and to decrease the luminance during the nighttime or tunnel traveling. In particular, the capability to adjust the luminance to a lower level is required in order to cope with users (such as westerners) who have pale pupil color.
For example, a dimming ratio (=maximum luminance: minimum luminance) of an LED included in a backlight device provided on an in-vehicle display is required to be, e.g., 20,000:1. For example, when the LED is rapidly dimmed, a problem such as instantaneous flickers may occur in the LED.
In addition, the number of LED light sources used for vehicles is increasing year by year, and electric heating lamps have been replaced in the related art. However, as the electric heating lamps are replaced by the LEDs, there has been a concern of malfunctions such as instantaneous flickering that has not been a problem in the past.
Instantaneous flickering of an LED is disclosed in the related art. In the related art, a technique includes a power supply line connected to a power terminal, a light emitting diode inserted into the power supply line, a switch connected to the power supply line at a downstream side of the light emitting diode to switch ON/OFF of the light emitting diode, a capacitor connected in parallel to the switch, a bypass line for bypassing the light emitting diode between the power terminal and the switch, a bypass line for bypassing the light emitting diode between the power terminal and the switch, and a switching part for selectively switching a supply source of electric power input to the power terminal to the power supply line or the bypass line. The switching part connects the power terminal and the bypass line when the light emitting diode is turned off. With this configuration, countermeasures for instantaneous flickering when an LED is turned on or turned off are taken.
FIG. 6 illustrates a configuration of a light emitting element driving apparatus LED driver IC 300 which has been reviewed in advance by the present inventor concerning a malfunction such as instantaneous flickering.
An output stage of the light emitting element driving apparatus installed outside the LED driver IC 300 has a switching element M1, a rectifying diode D1, a coil L1, an output capacitor C2, and a resistor R2. A well-known switching power source is constituted by a circuit configuration of these elements, and a PWM comparator, an error amplifier, a drive amplifier and the like which will be described later.
The LED driver IC 300 includes a Schmitt trigger 1, a PWM width detector 2, an overcurrent protection circuit 3, a buffer amplifier 4, a slope voltage generating part 5, a PWM comparator 6, a buffer amplifier 7, an error amplifier 8, a drive amplifier 9, an LED voltage selector 10, a comparator 11, a logic circuit 12, a constant current source 13, a switch SW1, and a switch SW2, and is configured by integrating them. In addition, the LED driver IC 300 has terminals PWMIN, CS, FB, LED1 to LED4, SWOUT, and COMP for establishing electrical connection with the outside.
A power source voltage VCC is connected to one end of the coil L1, and a drain of the switching element M1 formed of an n-channel MOSFET and an anode of the rectifying diode D1 are connected to the other end of the coil L1. A source of the switching element M1 is connected to one end of the resistor R2. The other end of the resistor R2 is connected to a ground potential GND. A gate of the switching element M1 is connected to the switching terminal SWOUT of the LED driver IC 300. One end of the output capacitor C2 is commonly connected to a cathode of the rectifying diode D1, and the other end thereof is connected to the ground potential GND. The common connection point corresponds to an output terminal OUT. An output voltage VOUT is generated at the output terminal OUT. Respective anodes of element arrays LE1, LE2, LE3, and LE4 in which a plurality of light emitting elements are connected in series are commonly connected to the output terminal OUT. The element arrays LE1, LE2, LE3, and LE4 constitute one light emitting element light source LE.
The output voltage VOUT is supplied as a driving voltage for driving the light emitting element light source LE. Voltage dividing resistors ROVP2 and ROVP1 are connected in series between the output terminal OUT and the ground potential GND. A connection point between the resistors ROVP2 and ROVP1 is connected to the buffer amplifier 7 via the feedback terminal FB of the LED driver IC 300. An output of the buffer amplifier 7 is connected to a first non-inverting input terminal (+) of the error amplifier 8 via the switch SW1.
Respective cathodes of the element arrays LE1 to LE4 are connected to the LED voltage selector 10 and the constant current source 13 via the LED connection terminals LED1 to LED4 of the LED driver IC 300.
An output of the LED voltage selector 10 is connected to one end of the switch SW2 and a non-inverting input terminal (+) of the comparator 11.
The other end of the switch SW2 is connected to a second non-inverting input terminal (+) of the error amplifier 8. A first reference voltage VREF1 is connected to an inverting input terminal (−) of the error amplifier 8. A second reference voltage VREF2 is connected to an inverting input terminal (−) of the comparator 11. The second reference voltage VREF2 is set slightly higher than the first reference voltage VREF1.
The error amplifier 8 amplifies a difference between one of a feedback voltage Vfb obtained by dividing the output voltage VOUT by the resistor ROVP2 and the resistor ROVP1 and an output voltage of the LED voltage selector 10 and the first reference voltage VREF1 to generate an error voltage Vcomp, and is connected to an inverting input terminal (−) of the PWM comparator 6 and the error amplifier output terminal COMP of the LED driver IC 300. One end of the resistor R1 is connected to the error amplifier output terminal COMP of the LED driver IC 300, the other end of the resistor R1 is connected to one end of the capacitor C1, and the other end of the capacitor C1 is connected to the ground potential GND. A phase characteristic of the LED driver IC 300 is set by the resistor R1 and the capacitor C1.
The LED voltage selector 10 outputs the lowest voltage with respect to the ground potential GND among the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4. In other words, it outputs the highest voltage (the largest voltage) of light emitting element drop voltages VLD which are drop voltages in the element arrays LE1 to LE4.
The comparator 11 compares the output voltage of the LED voltage selector 10 with the second reference voltage VREF2 to output the comparison result to the logic circuit 12.
The logic circuit 12 receives the output of the comparator 11, and outputs a first control signal LSDET1 after processing in the logic. The first control signal LSDET1 controls switching of the switching element M1 via the drive amplifier 9 so that each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 does not decrease.
A connection point between the source of the switching element M1 and one end of the resistor R2 is connected to the overcurrent protection circuit 3 and the buffer amplifier 4 via the protection input terminal CS of the LED driver IC 300. The overcurrent protection circuit 3 forcibly turns off the switching element M1 when a coil current flowing through the coil L1 reaches a predetermined overcurrent set value. An output of the buffer amplifier 4 is input to the slope voltage generating part 5. The slope voltage generating part 5 generates a sawtooth-shaped or triangular wave slope voltage Vslope. The slope voltage Vslope is connected to a non-inverting input terminal (+) of the PWM comparator 6. The PWM comparator 6 compares the error voltage of the error amplifier 8 with the slope voltage Vslope to output the comparison result to the drive amplifier 9.
The drive amplifier 9 controls the gate of the switching element M1 via the switching terminal SWOUT of the LED driver IC 300 based on the comparison result of the PWM comparator 6.
A pulsed PWM signal is input from a signal source (not shown) to the terminal PWMIN of the LED driver IC 300. The input PWM signal passes through the Schmitt trigger 1 which generates a dimming signal PWM and inputs the same to the PWM width detector 2. The PWM width detector 2 measures an ON width of the input dimming signal PWM, and outputs a high level H when it is equal to or greater than a threshold value and outputs a low level L as a dimming signal width detection signal PWM_WDET when it is less than the threshold value. The dimming signal width detection signal PWM_WDET is connected to the switch SW1, the switch SW2, and the error amplifier 8 to switch (ON/OFF) each of the switches, SW1 and SW2. The error amplifier 8 is always turned on when the switch SW1 is turned on, and operates synchronously with PWM when the switch SW2 is turned on.
In reviewing the LED driver 300, the present inventor attempted to develop a method of switching between two modes, namely a first feedback control mode and a second feedback control mode, depending on the ON width of the dimming signal PWM. That is, when the ON width of the dimming signal PWM is less than a threshold value, the switch SW1 is turned on and the switch SW2 is turned off, and the first feedback control mode is enabled. When the ON width of the dimming signal PWM is equal to or greater than the threshold value, the switch SW1 is turned off and the switch SW2 is turned on, and the second feedback control mode is enabled.
The reasons for adopting the first feedback control mode and the second feedback control mode are as follows.
When controlling is performed only in the first feedback control mode, it is necessary to set the output voltage VOUT equal to or higher than a specified value because it cannot be specified how much the light emitting element drop voltage VLD is generated in the light emitting element light source LE, and the voltage applied to the light emitting element light source LE becomes larger, increasing heat generation.
In addition, when controlling is performed only in the second feedback control mode, when a PWM pulse width becomes narrower, the time for switching the switching element M1 becomes shorter, energy cannot be accumulated in the coil, and the output voltage VOUT gradually decreases, making it impossible to supply a sufficient voltage to the light emitting element light source LE.
Further, even when both the first feedback control mode and the second feedback control mode are used, it was also recognized that the above malfunction cannot be solved when such combination is released, i.e., when the second feedback control mode is adopted when the ON width of the dimming signal PWM is less than the threshold value, and the first feedback control mode is adopted when the ON width of the dimming signal PWM is greater than the threshold value.
When the switch SW1 is turned on and the switch SW2 is turned off, the feedback voltage Vfb obtained by dividing the output voltage VOUT by the resistor ROVP2 and the resistor ROVP1 is input to the error amplifier 8. Therefore, the first feedback control mode in which the feedback voltage Vfb is used as a feedback signal is performed, and the output voltage VOUT is controlled to be constant.
On the other hand, when the switch SW1 is turned off and the switch SW2 is turned on, the minimum voltage on the cathode side with respect to the ground potential GND among the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 is input to the error amplifier 8. Therefore, the feedback control in which the minimum voltage on the cathode side of the element arrays LE1 to LE4 is used as the feedback signal is performed. A control method for controlling the output voltage VOUT based on the minimum voltage on the cathode side of the element arrays LE1 to LE4 is the second feedback control mode of the present disclosure.
By combining the first feedback control mode and the second feedback control mode, the dimming of the element arrays LE1 to LE4 corresponding to a duty of the dimming signal PWM can be performed.
FIG. 7 is a diagram illustrating a malfunction existing in the LED driver IC 300 illustrated in FIG. 6. FIG. 7 illustrates a timing chart when switching from a first feedback control mode to a second feedback control mode.
The dimming signal PWM indicates a state (first feedback control mode) in which the ON width of the dimming signal PWM is less than a threshold value of the dimming signal width detection signal PWM_WDET from time t0 to time t2, and indicates a state (second feedback control mode) in which the ON width of the dimming signal PWM is equal to or greater than the threshold value of the dimming signal width detection signal PWM_WDET after time t2.
The feedback voltage Vfb generated at the feedback terminal FB of the LED driver IC 300 is controlled in the first feedback control mode from the time t0 to the time t2. After the time t2, the feedback voltage Vfb is not controlled. After the time t2, the second feedback control mode is performed.
The output voltage VOUT has a value expressed by the following equation from the time t0 to the time t2.VOUT=(ROVP1+ROVP2)/ROVP1*VREF1
In the above equation, the resistor ROVP1 and the resistor ROVP2 are external resistors of the feedback terminal FB, and the first reference voltage VREF1 is a voltage generated in the LED driver IC 300.
A period from the time t2 to time t8 is a transient period before the output voltage VOUT is kept at a predetermined magnitude, and for example, at time t6, a potential difference between the minimum voltage of each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to the LED4 and the ground potential GND is indicated by Vcd5, which is lower than the first reference voltage VREF1 and does not become a sufficient voltage source for driving the constant current source 13. Therefore, a predetermined current cannot be supplied to the light emitting element light source LE. After the time t8, it is a value expressed by the following equation. Here, Vfmax indicates the largest light emitting element drop voltage VLD among the element arrays LE1 to LE4.VOUT=Vfmax+VREF1
Each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 when the dimming signal PWM has a low level L is expressed by the following equation from the time t0 to the time t2.VLED1 to VLED4=(ROVP1+ROVP2)/ROVP1*VREF1
At the time t1 when the LED is turned on, it is expressed by the following equation.VLED1 to VLED4=(ROVP1+ROVP2)/ROVP1*VREF1−Vfmax
Each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 corresponds to a transient period before it is kept at a predetermined magnitude from the time t2 to the time t8. Each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 is controlled to be the first reference voltage VREF1 at the time t8.
The first control signal LSDET1 indicates a low level L from the time t0 to the time t3. When each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 becomes equal to or lower than the second reference voltage VREF2 at the time t3, it is output as a high level H. The second reference voltage VREF2 is set slightly higher than the first reference voltage VREF1. The first control signal LSDET1 serves as an enable signal for switching the switching element M1 in the second feedback control mode. In the second feedback control mode, the LED driver IC 300 allows switching of the switching element M1 when the dimming signal PWM has a high level H and the first control signal LSDET1 has a high level H.
Respective currents ILED1 to ILED4 of the connection terminals LED1 to LED4 are stably supplied from the time t0 to the time t5. However, from the time t5 to the time t7 when each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 is low, the constant current source 13 deviates from a proper operation range, making it impossible to stably supply each of the currents ILED1 to ILED4 that should be supplied to the element arrays LE1 to LE4. In particular, at the time t6, the potential Vcd5 as a voltage source to be supplied to the constant current source 13 as described above becomes extremely small and a predetermined current cannot be supplied to the element arrays LE1 to LE4. Thus, a malfunction such as so-called instantaneous flickering occurs. Thereafter, each of the currents ILED1 to ILED4 is stably supplied at the time t8.
The voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is a voltage output from the error amplifier 8. At the times t0 to t2 when it operates in the first feedback control mode, when the currents ILED1 to ILED4 of the connection terminals LED1 to LED4 are averaged, the voltage is small and relatively low (close to the slope voltage) to be able to switch asynchronously with the dimming signal PWM.
The voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is kept as a voltage at which the error amplifier 8 goes to a high impedance state from the time t2 to the time t3 when each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 becomes equal to or lower than the second reference voltage VREF2.
By allowing each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 to become equal to or lower than the first reference voltage VREF1 at the time t4, the voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 starts to be charged.
The voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is charged up to the slope voltage or more at the time t6.
The charging of the voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is completed after the time t8.
The switching voltage Vsw is output to the switching terminal SWOUT to drive the switching element M1. The switching element M1 is switched off when the voltage of the feedback terminal FB exceeds the first reference voltage VREF1 at the times t0 to t2, and the switching element M1 is switched on and operates asynchronously with the dimming signal PWM when the voltage is lower than the first reference voltage VREF1.
When the width of the dimming signal PWM exceeds a predetermined threshold value at the time t2, the switching is made from the first feedback control mode to the second feedback control mode. Each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 gradually decreases, and the switching element M1 is switched off before the voltage becomes equal to or lower than the second reference voltage VREF2.
By allowing each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 to become equal to or lower than the second reference voltage VREF2 at the time t3, the first control signal LSDET1 becomes a high level and the switching element M1 of the LED driver IC 300 is switched on.
Since the voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is lower than the originally required operating point from the time t4 to the time t6, the switching element M1 of the LED driver IC 300 is not switched on.
By allowing each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 to become equal to or lower than the first reference voltage VREF1 at the time t4, the voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 starts to be charged. Thus, the voltage Vcomp of the error amplifier output terminal COMP of the LED driver IC 300 is charged up to the operating region at the time t6, and the switching element M1 of the LED driver IC 300 is switched on.
In a method of the related art, countermeasures are taken at the time of turning on and off, but nothing is disclosed about instantaneous flickering at the time of dimming.
In addition, it was found that the circuit of FIG. 6 which has been reviewed in advance before the present disclosure cannot stably supply the LED driving current from the time t5 to the time t7 in FIG. 7. Since this causes the voltage Vcomp of the error amplifier output terminal COMP to become lower than the slope voltage Vslope at the times t4 to t6 in FIG. 7, the switching element M1 cannot be sufficiently switched and each of the voltages VLED1 to VLED4 of the LED connection terminals LED1 to LED4 decreases and an appropriate voltage cannot be supplied to the constant current source 13. Therefore, an instantaneously flicker malfunction in the LED1 to LED4 was found.